Modern and Palaeozoic iron ooids - a similar volcanic origin

The discovery of an unconsolidated deposit of iron ooids of a kind not previously described, and in the vicinity of a volcanic island in Indonesia has shed new light on the genesis of many ancient iron oolites. The discovery shows that iron ooid formation is geologically a rapid process. The ooids form by chemical precipitation of cryptocrystalline iron oxyhydroxides on available grains on the seafloor, from seawater enriched with Fe, Al and Si. The enrichment can be a result of hydrothermal fluids, volcanic ash falls into shallow basins or rapid weathering of fresh volcanic rocks. Ordovician iron ooids from northern Europe are strikingly similar to the modem ooids in chemical composition, REE distributions and internal structures. The differences in mineralogy can be explained as diagenetic transformation reactions under different environments. A model for the formation of the Ordovician iron ooids in northern Europe from volcaniclastic material is presented, which could be applicable to other ancient oolites in the world. (C) 2000 Elsevier Science B.V. All rights reserved.